The present invention relates to a capacitive sensor having a diaphragm structure and designed to capacitively detect a change in pressure to be measured.
Generally, in a capacitive sensor, in order to minimize the parasitic capacitance formed between a pair of electrodes constituting a sensing capacitor portion, an insulating material must be used for at least one of the substrates.
As a pressure sensor of this type, a capacitive pressure sensor has been proposed (C. Y. Lee et al., "Quartz Capsule Pressure Transducer for the Automotive Industry", Society of Automotive Engineers, Inc., 1980). As shown in FIG. 1A, in this sensor, a quartz glass or sapphire substrate 2 having a stationary electrode 1, and a quartz glass or sapphire substrate 4 having a movable electrode 3 are arranged such that the surfaces of their electrodes oppose each other, and peripheral portions of the substrates 2 and 4 are bonded to each other with a low-melting glass 5 so as to have a predetermined gap G therebetween. As shown in FIG. 1B, a sensing capacitor portion 3s is formed at a central portion of the movable electrode 3, and a reference capacitor portion 3r is formed at a peripheral portion thereof.
As another pressure sensor of the same type, a capacitive pressure sensor having a diaphragm structure has been proposed (Japanese Patent Laid-Open No. 2-148768). As shown in FIG. 2, in this sensor, a cover glass 7 consisting of pyrex and having a stationary electrode 6, and a silicon wafer 9 having recess portions formed in its upper and lower surfaces and having a movable electrode 8 in the recess portion in the upper surface are arranged such that the surfaces of their electrodes oppose each other, and peripheral portions of the cover glass 7 and the silicon wafer 9 are bonded to each other through a bonding portion 10 by anodic bond.
However, according to the pressure sensor shown in FIGS. 1A and 1B, since the low-melting glass 5 is used to couple the quartz glass or sapphire substrate 2 to the quartz glass or sapphire substrate 4, which are arranged to oppose each other, the controllability of the gap G on the order of several .mu.m or less is poor. Therefore, in order to obtain a predetermined capacitance value, the size of each electrode must be increased, and the size of the pressure sensor is inevitably increased. In this case, for example, the size of the pressure sensor is as large as 25.4 mm.phi.. In addition, the controllability of the base capacitance is poor, resulting in considerable disadvantages in terms of productivity and cost.
Since the low-melting glass 5 different from the substrate material is used as the bonding material between the quartz glass or sapphire substrate 2 and the quartz glass or sapphire substrate 4, a stress is generated by the difference between the coefficients of thermal expansion of the substrate material and the low-melting material with a variation in measurement temperature. As the temperature dependence of the sensor sensitivity is increased, drift tends to occur.
A bonding operation using the low-melting glass 5 is performed at a temperature of about 300.degree. C. or more. When the resultant structure is cooled down to a working temperature after the bonding operation, a residual stress is generated. This stress gradually changes with time and hence adversely affects high-precision, reliable pressure measurement.
According to the pressure sensor shown in FIG. 2, the cover glass 7 and the silicon wafer 9 are made of different materials, i.e., pyrex and silicon. Although the coefficients of thermal expansion of pyrex and silicon are close to each other, they are not completely the same. For this reason, when the structure formed by bonding the cover glass 7 to silicon wafer 9 at a temperature of several hundreds degrees is cooled down to a working temperature, the stress remains in the bonding portion 10 between the cover glass 7 and the silicon wafer 9. Although this stress is small, since the stress changes with time, high-precision, reliable pressure measurement is adversely affected by the stress.